The presence of following chemical which cause efflorescence in cement is:
🌼 What is Efflorescence?
Efflorescence is a white, powdery deposit of crystalline salts that appears on the surface of porous materials like concrete, brick, or stone. It occurs when water-soluble salts within the material are carried to the surface by moisture. As the water evaporates, it leaves the salt deposits behind.
While often just a cosmetic issue, severe efflorescence can indicate underlying moisture problems and can sometimes cause spalling of the surface.
📝 Detailed Analysis of the Options
(a) Alumina, (b) Calcium oxide, (c) Silica
These are the three major chemical constituents of cement. They react with water to form the stable, insoluble compounds that give concrete its strength. They are not water-soluble in their hydrated state and therefore do not cause efflorescence.
(d) Alkalies
This is the correct answer. The primary source of efflorescence-causing salts in concrete is the presence of soluble alkali compounds in the cement. These are mainly soda (Na₂O) and potash (K₂O). These compounds can react with water and be transported to the surface. To control this, the total alkali content in cement is limited by standards, typically to less than 1% (and often less than 0.6% for low-alkali cement).
📊 Role of Minor Compounds in Cement Durability
| Compound | Typical % | Effect of Excess Amount |
|---|---|---|
| Alkalies (Na₂O, K₂O) | 0.5 - 1.3% | Causes efflorescence; can also cause alkali-aggregate reaction. |
| Magnesium Oxide (MgO) | 0.1 - 4% | Causes unsoundness (delayed expansion and cracking). |
| Sulphur Trioxide (SO₃) | 1 - 3% | Causes unsoundness if in excess. (Comes from gypsum). |
💡 Study Tips
- Efflorescence = Soluble Salts: The key to efflorescence is the presence of salts that can dissolve in water.
- Alkalies are the Culprit: In cement, the main soluble salts are the alkalies (soda and potash). Memorize this direct link.
- Look for the White Powder: Visually associate the term "efflorescence" with the white, powdery stains you sometimes see on brick walls or concrete.
- Control is Key: Remember that the amount of alkalies is limited in cement manufacturing to prevent this problem.